Isobutylene, the major source of tertiary-butyl groups, is produced primarily by catalytic cracking of gas oils which yields a mixture of butane and butylenes. In commercial processes, it is the common practice to separate isobutylene from other C.sub.4 -hydrocarbons obtained in the cracking process by absorption in sulfuric acid. The separation of isobutylene from the other compounds in the acid solution is costly. It is therefore economically advantageous to use a mixed feed containing isobutylene or a reaction product of isobutylene formed as a means for separating isobutylene from a mixture of C.sub.4 -hydrocarbons, as a starting material for the present process. For example, in the course of the isobutylene separation by sulfuric acid absorption, there is formed approximately 5-10% of di-isobutylene and triisobutylene. The use of an isobutylene mixture of this nature as a starting material in the present process offers a distinct cost advantage over the use of pure isobutylene.
The reaction of isobutylene to form butyl ethers represents a different means for separating isobutylene from a mixed C.sub.4 -stream, and the ether may be another source of a low cost starting material. Another source of the tertiary butyl group for the starting material of the present invention is tertiary-butyl alcohol which may be available as a low cost by-product from other processes, such as a by-product of epoxidation with tertiary-butyl hydroperoxide.
It is therefore an outstanding advantage of the present process that the C.sub.4 -hydrocarbon mixture resulting from the acid absorption or other tertiary-butyl group sources such as isobutylene derivatives, can be directly converted to methacrolein, methacrylic acid or methacrylonitrile without the need for separate regeneration and purification of isobutylene. In the course of the reaction of the present invention, the catalyst compositions utilized also promote the formation of isobutylene from the tertiarybutyl compounds employed in the reaction, and the isobutylene thus formed can be selectively oxidized or ammoxidized to the corresponding unsaturated aldehyde and acid or the unsaturated nitrile, respectively.